Three Twitter video clips capture the Ferguson gunfire and aftermath. Link to video.

A young black man is in a critical and unstable condition after being shot by police in Ferguson on the edges of a demonstration marking the first anniversary of the fatal shooting of Michael Brown, which was once again dispersed by police firing teargas.

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Tyrone Harris was shot by four plainclothes detectives after allegedly firing on their unmarked vehicle as he walked from a gunfight between several people lateon Sundaynight, according to the St Louis County police department.

“We can’t afford to have this kind of violence,” police chief Jon Belmar said at a 2.30am (8.30am BST) press conference on Monday. “There is a small group of people who are intent on making sure we don’t have peace.”

Harris, 18, was charged later on Monday with four counts of first-degree assault on law enforcement officials, five counts of armed criminal action and one count of shooting a firearm at a vehicle, a police department spokesman said, adding that Harris was being detained on a $250,000 cash-only bond.

Belmar stressed that the approximately six people involved in the exchange of fire that led to the shooting by police should not be associated with the demonstrations. “They weren’t protesters; they were criminals,” he said.

Police said five men, all from the St Louis area, were arrested. Among them Trevion Hopson, 17, and Jeffrey Pruitt, 27, were charged with unlawful use of a weapon. Three others were charged with interfering. One police officer suffered a cut to the face after being hit by a rock and another two officers were “pepper-sprayed by protesters”, according to a police spokesman.

A protester stands facing a police line in Ferguson shortly before shots were fired nearby. Photograph: Rick Wilking/Reuters

In the hours after the shooting, dozens of protesters were swept from a main street in the city by police who fired a barrage of gas and smoke grenades. Officers wearing body armour were backed by the military-style armoured vehicles seen after Brown’s death last August.

Harris’s father told the St Louis Post-Dispatch that his son was a graduate of Normandy high school, which Brown also attended, and that the pair had been close.

The police chief said the plainclothes detectives had been tracking Harris because they believed him to be armed. A young woman described by friends as the girlfriend of the injured man sat sobbing hysterically beside a row of shops opposite where he was shot. A large group of police in riot gear promptly marched towards the woman and her friends, driving them and protesters away from the scene as clergy peacekeepers pleaded with them to stand back from her.

The bursts of gunfire rang out at about 11.15pm near a block north of where protesters were squaring off with officers clad in riot gear for the first time during the largely peaceful anniversary weekend. Police, reporters and protesters gathered behind vehicles while others scattered. Dozens of shell casings were recovered by police.

The young man was seen by several reporters lying wounded and handcuffed on the ground beside officers. He was recorded on video by Tony Rice, a Ferguson-based protester and activist, who said he was arrested seconds after filming the scene for refusing to move back. A legal observer said Rice was released by police shortly after.

DeRay McKesson, a prominent protest leader, said he and several others ran north for cover after the shots were fired. “We heard bullets buzzing through the air,” he said. As reporters and photographers attempted to peer around vehicles, several officers ordered them to get down. “It’s not worth it,” one told the Guardian and journalists for other outlets.

Belmar said a 9mm Sig Sauer pistol that was stolen in Missouri in 2014 was recovered from the man who was shot.

St Louis County police later said two teenagers were shot at about 2.15am on Canfield Drive, the street where Brown was shot dead last year. Officer Shawn McGuire said in a statement that an “unknown black male wearing a red hooded sweatshirt started shooting at them from the rear passenger side of an unknown vehicle”.

A 17-year-old was shot once in the chest/shoulder area, according to police, while the 19-year-old was shot once in the chest. McGuire said both were treated at a local hospital for non-life threatening injuries.

A few lootings had been reported through the night and windows to some store fronts were smashed. Paul Hampel, a reporter for the St Louis Post-Dispatch, was assaulted and robbed by looters minutes after reporting on their actions on Twitter.

The armoured vehicles rolled back on to West Florissant Avenue minutes after the shooting. Officers from St Louis County, who had been policing the protest, were joined by troopers from the Missouri state highway patrol in riot helmets.

An hour after the shooting, a few dozen protesters and reporters were penned in by police at the car park in front of Ferguson Market & Liquor, the convenience store from which Brown was accused of stealing cigarillos minutes before he was shot.

Two police vehicles were shot at in the confrontation, according to police. St Louis County vehicles do not carry the dashboard cameras used by some other agencies. Belmar said the officers were not wearing body cameras because they were plainclothes detectives.

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Police began firing smoke and gas shortly before 2am as some protesters threw stones and bottles while dozens gathered at the intersection of West Florissant and Canfield Drive, the street where Brown was fatally shot last 9 August by Darren Wilson, a white Ferguson police officer.

“Failure to disperse may result in your detention, arrest, and/or exposure to chemical or less-lethal munitions,” a police officer said over a loudspeaker. “Police intend to use chemical munitions.” While some were swept down the main street others were shot by a bombardment despite retreating down the side street as requested by police.

US attorney general Loretta Lynch used a speech to police officers on Monday to “strongly condemn the violence against the community, including police officers, in Ferguson”.

“Not only does violence obscure any message of peaceful protest, it places the community, as well as the officers who seek to protect it, in harm’s way,” Lynch told the national Fraternal Order of Police. “The weekend’s events were peaceful and promoted a message of reconciliation and healing. But incidents of violence, such as we saw last night, are contrary to both that message, along with everything that all of us, including this group, have worked to achieve over the past year.”

A series of acts of civil disobedience were planned across the St Louis area on Monday including a march in the downtown city district. The so-called “Moral Monday” actions were intended to keep pressure on authorities following the passage of the anniversary weekend, organisers said.

Wilson was cleared of criminal wrongdoing in November by a state grand jury and the US Department of Justice declined to prosecute him for civil rights violations. Brown’s family is suing Wilson and the city, alleging they caused the wrongful death of their son.

Gunfire broke out late on Sunday night in a confrontation between riot police and protesters after a day of events commemorating the fatal shooting of unarmed black teenager Michael Brown by a white officer one year ago